The present invention relates to high-frequency continuous-time filters (of the type which comprises an input circuit portion having signal inputs and an output circuit portion), and more particularly to differential stages therefor.
The field of application of the disclosed innovative differential circuit is particularly, but not solely, related to monolithically integrated, time-continuous band-pass filters, and the description to follow will make reference to this field of application for convenience of illustration.
In recent years, several techniques have been proposed for making integrated time-continuous filters. Of these techniques, that using a transconductor.sup.1 stage for the filter base block is outstanding. FNT .sup.1 A transconductor is a voltage-controlled variable-transconductance stage. It is an integral part of the operational transconductance amplifier "OTA") which is a voltage-controlled current-source amplifier. Transconductors are used in active filters, and also in gyrators, oscillators, and circuits for impedance transformation. See generally J. Scott, ANALOG ELECTRONIC DESIGN (1991), which is hereby incorporated by reference. Some specific examples of the literature on transconductor designs, and their application to continuous-time filters, includes the following, all of which are hereby incorporated by reference: Silva-Martinez et al., "A large-signal very low-distortion transconductor for high-frequency continuous-time filters," IEEE JOURNAL OF SOLD-STATE CIRCUITS vol.26, no.7 p.946-55 (Jul. 1991); Tanimoto et al., "Realization of a 1-V active filter using a linearization technique employing plurality of emitter-coupled pairs," IEEE JOURNAL OF SOLID-STATE CIRCUITS vol.26, no.7 p.937-45 (Jul. 1991); Castello et at., "A very linear BiCMOS transconductor for high-frequency filtering applications," in the 1990 IEEE INTERNATIONAL SYMPOSIUM ON CIRCUITS AND SYSTEMS vol.2, pp.1364-7; Perry, "A flexible transconductor-capacitor filter demonstrator," in the 1989 IEEE INTERNATIONAL SYMPOSIUM ON CIRCUITS AND SYSTEMS vol.2, p.1075-8; Haigh et al., "Continuous-time and switched capacitor monolithic filters based on current and charge simulation," IEE PROCEEDINGS G (Circuits, Devices and Systems) vol.137, no.2 p.147-55 (1990); de Heij et at., "Transconductor and integrator circuits for integrated bipolar video frequency filters," 1989 IEEE INTERNATIONAL SYMPOSIUM ON CIRCUITS AND SYSTEMS vol.1 p.114-17; Perry, "An integrated continuous-time bipolar transconductor-capacitor filter," 24 IEEE JOURNAL OF SOLID-STATE CIRCUITS p.732-5 (June 1989); Nedungadi et al., "High-frequency voltage-controlled continuous-time lowpass filter using linearised CMOS integrators," 22 ELECTRONICS LETTERS no.14 p.729-31 (3 Jul. 1986); and Czarnul et at., "MOS tunable transconductor," 22 ELECTRONICS LETTERS no.13 p.721-2 (19 Jun. 1986).
Such a technique appears to be the most effective in high frequency applications. This is probably attributable to the fact that, in a transconductor filter, the voltage-to-current conversion phase takes place in an open loop, so that the so-called non-dominant pole of the converter is not restricted by the unitary frequency gain of an operational amplifier, as is instead the case with filters in the MOSFET-C technology.
However, transconductor filters have a drawback in that they are especially responsive to stray capacitances, particularly on account of being operated in an open loop fashion. This restricts their passband, especially at high frequencies.
The underlying technical problem of this invention is to provide a new transconductor differential stage which has such structural and performance characteristics as to overcome the drawbacks with which the prior art approaches have been beset, but without giving up any of their well-recognized advantages.
The preferred embodiment of the invention provides a differential stage using a mixed bipolar-MOS technology. The input portion comprises a pair of MOS transistors whose gate terminals coincide with said signal inputs, and the output portion comprises a pair of bipolar transistors arranged with their bases in common. The drain of each MOS input transistor is connected to the emitter of an npn bipolar. These two matched bipolars have their gates connected together with the gate of a third bipolar, which is diode-connected. Two matched current sources feed the two bipolars, and a third current source feeds the third bipolar. A single controlled current sink is connected to the sources of both MOS input transistors, and also (through a resistor) to the third bipolar.